Many heat absorption and/or heat storage applications have high heat loads, require precise temperature control, and/or have limited heat rejection options. Such applications include, but are not limited to, cooling microelectronics such as computer processors, cooling power conditioning equipment for prime power generation, cooling high-power laser diodes, cooling solid-state laser systems, cooling high power micro-wave systems, and cooling electronics in spacecraft applications.
Thermal energy storage devices typically include a housing, which encases a bulk volume of phase change material. Heat is conductively transferred from the heat source through the housing structure and begins to melt the phase change material, effectively storing the waste heat. Paraffin's are often selected as the phase change material in thermal energy storage devices because of their wide range of melting temperatures (−30° C. to 110° C.) and their high latent heat characteristics (150-250 kJ/kg). The disadvantage of most phase change materials is that these materials typically have a low thermal conductivity (k), especially in the liquid phase. Typical thermal conductivities are less than 0.2 W/m-K. This can result in large temperature gradients in the device being cooled in order to melt the entire volume of phase change material contained in the thermal energy storage device. In a typical phase change process a melting front is seen during the heat transfer process. The front is a dividing line between the melted material and the non-melted material. Heat is conducted across this front in order to melt the solid phase of the phase change material.
Some semiconductor devices, such as laser diodes, require the temperature control to be within ±2° C. and cannot tolerate large temperature increases during operation. In fact, with respect to this problem, the phase change material thermal conductivity is typically about 2-3 orders of magnitude too low. Due to the low thermal conductivity associated with these types of thermal energy storage devices these systems are limited in the rate at which they can absorb heat.